I purchased a Double J Pro Wide for my mare thinking it fit after trying one on her, but as I began riding her more consistently I noticed her withers starting to dip more and she became sore when palpated there. If I place just the saddle on her back, there are 'pockets' in the wither area under the saddle. I was riding her with a Biofit pad because she had small dips in her withers to begin with.

I did not realize that the Pozzi Pros had different gullet measurements than the Double J Pros. I know I need to go to a standard fit and honestly want to purchase a Pozzi, but I am unsure how to go about determining if a Standard Pro or Pozzi would fit her better. Does anyone know the specific measurements or have experiences with this?

I'm not an expert but took my hard to fit mare there to have her fitted. The difference in the Pozzi and the Pro is the swells and the cantle height (either can be built on multiple trees). I ended up getting the Pro in a #9 tree which has a little more flare in the shoulders.  If you can take your horse there it's very helpful.  It helped me to see the bare tree sitting on her back.  Then I tried several on and rode her in their arena.